1. Field of the Art
The embodiments relate, in general, to a process for purifying wood pulp fibers using a caustic-borate extraction to dissolve hemicellulose from the fibers. More particularly, the embodiments relate to a caustic-borate extraction process including a method for recovering the caustic and the borate from the hemicaustic effluent stream, using a nanofiltration membrane.
2. Description of Related Art
It is well known to those of ordinary skill in the art of wood pulp processing to use an alkaline treatment process, such as “mercerization,” “cold caustic treatment,” or “cold caustic extraction.” Cold caustic extracted fibers may be used in a variety of applications, including as a raw material in the production of regenerated fibers, such as rayon or cellulose acetate. Cold caustic extraction beneficially reduces the hemicellulose content of cellulosic fibers. Hemicellulose is generally defined as low molecular weight polysaccharaide units inherently present in all cellulosic fibers. Hemicellulose is particularly problematic in the production of regenerated fibers, because the hemicellulose clogs spinneret holes during fiber formation.
In general, alkaline treatments such as cold caustic extractions are performed by mixing a low consistency cellulose slurry with an alkaline solution, and allowing the mixture to extract for a suitable amount of time for the hemicellulose to be dissolved and diffuse out of the fiber walls. The extracted cellulose fiber is subsequently washed to remove the caustic solution and the dissolved hemicellulose, and then may be subjected to downstream processing. The effluent stream produced during washing, aptly referred to as a “hemicaustic” stream, generally contains a mixture of extracted hemicellulose, alkali metal hydroxide and water.
The handling and disposal of hemicaustic streams traditionally have been problematic. While the hemicaustic streams may be used in other pulp production processes, they are of limited use, and their disposal poses environmental concerns. The hemicaustic effluent streams and alkali metal hydroxide solutions associated with alkaline treatment process typically require specialized handling procedures to ensure that such materials are not discharged to the environment. In addition to environmental concerns, the economic value of the hemicellulose contained within the hemicaustic stream is lost if the hemicellulose is not recovered. As such, methods have been disclosed for the recovery of hemicellulose and alkali hydroxides from the hemicaustic stream. For example, U.S. Pat. No. 6,896,810 to Ali et al., which is hereby incorporated by reference in its entirety, proposes a system and process by which a portion of the alkaline solution may be recovered from the hemicaustic effluent stream and recycled back into the alkaline treatment system. The process includes a nanofiltration system to remove the hemicellulose from the spent caustic extraction liquor, i.e., the hemicaustic.
It has been disclosed that the addition of boron compounds to the caustic extraction solution further enhances the extraction of impurites from wood pulps. For example, U.S. Pat. No. 3,068,141 to MacClaren, et al., which is hereby incorporated by reference in its entirety, discloses the preparation of a refined cellulose product from a kraft wood product. The refining procedure includes treating the wood pulp using a strong aqueous alkali, such as aqueous sodium hydroxide, containing a boron compound, such as boric acid or Na2B4O7, followed or preceded by an extraction procedure using dilute alkali solution at high temperatures. U.S. Pat. No. 3,305,433, to Partlow, which is hereby incorporated by reference in its entirety, discloses a cold caustic refining treatment for cellulose that includes a soluble borate in a caustic solution. The solution inhibits the mercerizing effect of the concentrated cold caustic solution without adversely affecting its solvent power for hemicellulose and other impurities.
However, despite the potential advantages of the caustic-borate wood pulp extraction process these treatment processes have not been commercialized, to date, because there does not currently exist a viable use for the caustic-borate hemicaustic effluent produced, nor a viable method for recovering both the NaOH and the borate from the hemicaustic effluent. Traditionally, hemicaustic effluent streams have been recycled as soda makeup in the kraft pulping processes, or in alkaline extraction stages in a bleaching operation. However, having borate in the hemicaustic stream creates many processing problems, rendering them unsuitable for use in either process. And while methods for recovering caustic from the hemicaustic stream have been disclosed, none of these methods addresses the recovery of caustic-borate solution from a hemicaustic effluent stream. Indeed, the use of borate complicates the recovery of sodium hydroxide from hemicaustic effluent. See Pulp and Paper Manufacture, Vol. 4: Sulfite Sci. & Tech., 238 (M. J. Kocurek et al. eds., 1985). Because there has been no efficient method for recovering and recycling the costly caustic-borate solution, the caustic-borate extraction systems have not been commercialized.
The description herein of certain advantages and disadvantages of known processes, methods, and treatment compositions, is not intended to limit the scope of the embodiments. Indeed, the embodiments may include some or all of the processes, methods, and treatment compositions described above without suffering from the same disadvantages.